A DMOS transistor is known as one of power semiconductor devices.
The DMOS transistor comprises a drift region adjacent to a drain diffusion layer having a high impurity concentration. The drift region has the same conductivity type as that of the drain diffusion layer, and has a lower impurity concentration than the drain diffusion layer. The DMOS transistor is characterized in that its switching speed is fast in a relatively-low voltage region, and its conversion efficiency is high. The DMOS transistor may perform an operation with a high breakdown voltage and a low ON resistance may be achieved at the same time. However, in such a DMOS transistor, an element termination region is formed at an end region of an element region where a DMOS transistor itself is formed. In some cases, an element termination region does not have a predetermined breakdown voltage even when an element region has such a predetermined breakdown voltage.
In this case, the breakdown voltage of the whole element is determined by the breakdown voltage of the element termination region. With the conventional DMOS transistor, concentration of an electric field occurs in such an element termination region, and impact-ion due to this may easily be generated. As a result, a breakdown voltage of the whole semiconductor device becomes lower. Therefore, a semiconductor device having an element termination region with a high breakdown voltage is required. On the other hand, it is highly requested that a circuit area of the whole semiconductor device is decreased.